Group VIB transition metals include, for purposes of this application, tungsten, molybdenum and chromium. The Group VIB, transition metal elements, such as tungsten, molybdenum, and chromium, have characteristics that allow their incorporation into some new, high-performance alloys. Their high stiffness suggests that they have intrinsic high strength. This indicates that they should have high fracture energy and high specific resilience. It also suggests that they are capable of being made into hard and wear resistant alloys. They have high melting temperatures, as well. Unfortunately, the potentially superior mechanical properties of these materials are seldom realized because of their lack of toughness.
They are all used as pure metals and as important alloying species with base metals. As pure metals, or as the major species in alloys, tungsten and molybdenum are more important than chromium as structural materials. Chromium is used more frequently as a coating.
These Group VIB transition metals, such as tungsten, are used industrially as pure metals, sometimes containing small quantities of a finely divided dispersant; as an alloy with other high melting metals; or as a pure metal cemented into a body with small quantities of a lower melting metal matrix; or as a carbide, either pure or alloyed, cemented with a similar lower melting metal matrix. They are also used as a dilute alloying species in high strength and high hardness base metal alloys.
Among the most important applications of tungsten, for example, are for resistance wire as in lamp bulbs and vacuum tubes, extremely small conductors in microprocessors, x-ray targets, so-called heavy metal alloys, and cemented carbide tool and wear parts. The wire and x-ray target uses take advantage of tungsten's high melting temperature; the microprocessor use of its electrical conductivity and thermal expansion coefficient; the heavy metal alloys of its high specific gravity; and the cemented carbides of the hardness and wear resistance of its monocarbide.
In most instances, it is important for these Group VIB transition metals to have the highest strength and toughness, consistent with the maintenance of its other important properties.
Fine tungsten wire, for example, after the large amount of mechanical work which goes into its manufacture, exhibits high strength. Bulk metal parts of tungsten are usually much weaker, however. In all but a few instances, e.g., the fine wire, tungsten parts suffer from lack of toughness. Even the wire soon loses both strength and ductility on heating due to the work being a high driving force for re-crystallization and grain growth. The brittleness of x-ray targets and other larger bodies has been avoided, at considerable increase in cost, by the addition of the rare metal, rhenium, as an alloying species in quantities as high as twenty-five percent.
The heavy metal and cemented carbide parts rely on another approach to achieve acceptable toughness. They are made by pressing and sintering a mixture of pure metal powder, or of carbide powder, with a lower-melting, more ductile, base metal. The tungsten or tungsten carbide is thereby cemented by the small quantities of the ductile base metal.
Properties of the final product are achieved by the judicious selection of the matrix metal composition, the size of the metal powders, or the size and composition of the carbide powders. For many applications of tungsten and for most applications of tungsten carbide the base-metal-cemented, these pseudo-alloys are the only practical solutions. There are many instances, however, where the incorporation of the softer, lower-melting, less-stiff, and less corrosion-resistant cement substantially degrades the usefulness of the bodies. Pure tungsten, or alloys of tungsten with strengthening or hardening species which would not use such cement would be much more useful.
With regard to metals and other materials in general, it has been well known to materials engineers and scientists that refinement of the crystal habit of bodies increases yield strength, and hardness. Since ancient days mechanical working to reduce their grain size has strengthened metal parts. With more sophisticated understanding, the so-called Hall-Petch relationship has become generally accepted. This relationship teaches that the yield strength of materials varies inversely with the reciprocal of the square root of the grain size. In a more recent publication, Jundal and Armstrong (see Trans. AIME 1969 vol. 245, pg. 625) reported that the Hall-Petch relationship could be extended to treat the increase in material hardness with grain size reduction as well as yield strength. Additional verification, for the case of the hardness of tungsten, comes from Vashi, et al. (see Metallurgical Trans., Vol. 1, June 1970, pg. 1769-1771). (The entire contents of all publications and patents mentioned anywhere in this disclosure are hereby incorporated by reference.)
Within the last decade, research has demonstrated that the dramatic effects on properties can be extended in materials of much finer grain refinement than had been earlier possible. Progress in the manufacture of cemented tungsten carbide cutting tool materials discussed above is a particularly good example of such improvement. Two decades ago the most modern of these cemented carbides had WC crystallite sizes no smaller than about two microns. Today, they are made quite regularly, commercially, with 0.4 micron (400 nm) crystals; and even smaller, on an experimental basis. This has resulted in superior products from the point of view of strength and wear resistance.
This reduction in grain size is not accomplished without difficulty. There are practical limits to the fineness of powders which may be used in the pressing and sintering process. Very small powders have long been considered explosion and worker-ingestion hazards. Even more importantly, these powders tend to agglomerate in handling, thereby preventing the formation of a final product with a crystal refinement as small as might be desired.
Advances to reduce the agglomeration problems have been claimed to be effected by the use of a spray-reaction process from salts of tungsten and the matrix metal with subsequent gas-phase carburization. This process is described in U.S. Pat. Nos. 5,230,729 and 5,352,269. Further, however, even after these very fine powders have been pressed successfully to a so-called green body, there is a tendency toward grain growth upon sintering, although efforts have been made to alloy the cementing metals to allow lower temperature processing and to minimize this grain growth. This approach is described in U.S. Pat. No. 5,841,044.
For reasons which have not been totally explained, none of sub-micron-size or nanostructure cemented carbides, except those with grain sizes above about 0.4 μm, or even above 0.8 μm, has shown sufficiently good toughness to be generally accepted commercially.
In the materials science arena, however, investigators have become increasingly anxious to investigate the effects of nano-technology. Nano-technology is usually defined as dealing in microcrystalline sizes below 0.1 μm (100 nm). Because of the aforementioned limitations, and because they need only small samples, they have chosen to use deposition techniques to make their research samples. Deposition is an attractive way to make extremely fine-grain materials since the crystallites of the materials of interest may be grown and consolidated, simultaneously, at temperatures which are low relative to their fusion temperatures, or even to their sintering temperatures. These bodies made by various deposition methods, therefore, need not be limited as to their coarse crystalline habit, as in casting; or as to agglomeration, or grain growth, as in powder pressing and sintering. Properly manipulated, they can be consolidated to virtually full density, quite free of internal voids and defects.
Both electrochemical deposition (ECD) or physical vapor deposition (PVD) techniques have been used by these scientists to make such samples for their scientific investigations. In the present application, physical vapor deposition refers to any of the group of similar methods, including evaporation, reactive evaporation, sputtering, reactive sputtering, and ion-plating. Such efforts are described in papers by Menezes and Anderson; J. Electrochemical Soc. 137, 440 (1990) and Chu and Bamett; J. Appl. Phys., Vol 77, No. 9, 1 May 1995. The samples have been useful to investigate the achievable improvement in properties from materials with grain refinement smaller than 0.25 micron (<250 nanometers). Small-scale samples have been made and tested. They have usually been made of a multiplicity of thin layers. Films with crystallite sizes well below 100 nm (even below 10 nm) have been successfully synthesized. These techniques, however, approach the objects of high performance materials in a very different way from those of the invention. They did not involve the strengthening and hardening of a metal with some intrinsic toughness, but rather an investigation of what happens when the grains of an intrinsically brittle material are refined.
It has been determined that much greater hardness can be achieved in such materials by the aforementioned techniques. However, improvements in strength or toughness have not been generally measured and reported.
The chemical vapor deposition (CVD) process would be more appropriate for the manufacture of industrial parts of the materials of interest than ECD or PVD. CVD, although requiring processing temperatures higher than either ECD or PVD, can still be processed well below the required fusion temperatures or sintering temperatures for the materials of interest. In the present application chemical vapor deposition is meant to include both simple thermally-activated CVD as well as plasma-assisted CVD. Since the control of CVD is more difficult than that of either ECD or PVD, it has been used very sparingly for any kind of nanotechnology research and hardly at all for any commercial manufacture of such fine-grain materials.
There are three notable exceptions. The most significant one is described in U.S. Pat. No. 4,162,345 to Holzl ('345). Two decades ago, the inventor, Holzl (one of the co-inventors of the current invention) taught, in the '345 patent, that materials made by a then-unique variation of the CVD process could be made to demonstrate a useful combination of strength and hardness such as to provide excellent wear resistance. The material could be described as an early version of what is currently being called a nanostructure.
The second is a research program conducted at Stevens Institute of Technology by Eroglu and Gallois in which thin nanostructure TiN/TiC coatings were investigated (see “Design and Chemical Vapor Deposition of Graded TiN/TiC Coatings”; Surface and Coatings Technology 49, 275 (1991)). Like the Chu and Barnett work, cited above, these investigators took a different approach than that of the invention. They were also investigating the refinement of normally brittle materials.
The third is a wear-resistant coating for cemented carbide tools which has been offered commercially since late 1998 by Widia Valenite. That company introduced a thin, nanocrystalline coating for cutting tools made by what is called multilayer CVD (MLCVD). They report improved wear life for certain cutting applications and claim that crack formation through the entire thickness of the coating is minimized by the multilayer configuration. The reported coating is comprised of conventional, brittle, coating materials, titanium nitride and titanium carbonitride. No improvement in strength was reported, or should have been expected from this work.
The background art closest to the current invention is the referenced work of the '345 patent. Most of the microcrystallites in the Holzl material were in the order of 50-100 nm, but it contained some that were as much as ten times larger. The material was actually used for certain important valve trim in the NASA space program. Unfortunately, the method of the '345 patent could not be reproduced with acceptable reliability and was extremely expensive. The irregularity of the crystallite size was a major problem which was never adequately solved. The process was subsequently discarded as unacceptable for industrial use.
However, there is ample reason to continue to be interested in CVD as a process for making nanostructural parts. Electrochemical deposition while totally acceptable for the common metals is practically useless for making refractory materials such as tungsten, its alloys, or compounds. PVD can be used to make common metals and compounds at very high rates, but for refractory metals and alloys, deposition rates are unacceptably low. Chemical vapor deposition, on the other hand, can synthesize such refractory metals and ceramics at very acceptable commercial production rates. CVD is superior to ECD and PVD as manufacturing processes in other ways, as well. Principal among them are its excellent throwing power and its ability to make materials of higher and essentially full density, virtually free of internal voids. The use of CVD to produce high melting and chemical and wear-resistant metals and ceramics is well-known.
Processes for making free-standing shapes of the so-called refractory metals and alloys have been known for decades. For example, pure tungsten tubing has been made commercially by depositing the metal on a mandrel from which it is then removed. Parts of good purity exhibit a Vicker's hardness of about 4 GPa. Utilizing the method of Cahoon et al. (see J. B. Cahoon, W. H. Broughton, and A. R. Kutzak, Metallurgical Transactions, vol 2, pp. 1979-1983, 1971), which teaches that the yield stress of a material that is fully strain-hardened is approximately equal to one third the Vicker's hardness, a maximum yield stress for high purity, CVD tungsten of 1300 MPa would be predicted. This value is an upper anticipated limit as the CVD tungsten is not fully strain-hardened. In practice, maximum values of 900 MPa can be obtained and the material displays limited ductility at room temperature. The columnar growth of the CVD tungsten produces near-continuous grain boundaries, which act as a volumetric flaw within the material. This structure leads to brittle failure of the tungsten at room temperature with strengths closer to 300 MPa for larger-grained, free-standing shapes. The corresponding low fracture toughness limits their utility. Reducing grain size would be expected to increase the strength. One technique for so doing is the lowering of the deposition temperature. The process then suffers from reduced deposition rates. A second technique involves mechanical burnishing the work piece during deposition (see L. W. Roberts; Proc., Sixth Plansee Seminar, Reutte, Austria, 1967; pp 881-884). This is mechanistically difficult on all but the simplest of work pieces. The highest strengths achievable by either of these techniques is, at most, about 900 MPa. All of the strength values cited above, and those that follow in this application, are flexural strengths, measured in 3-point bending with 2-4 mm diameter round rods.
Brittle materials like the refractory carbides, nitrides, borides and suicides are also conveniently made by CVD. CVD-synthesized tungsten carbide, having a hardness above 20 GPa, is not likely to have a strength of greater than 70 MPa and is essentially useless as anything but adherent thin coatings.
CVD has been used for years to produce thin films, such as oxidation-resistant coatings for high temperature metals and wear-resultant coatings for a wide variety of cutting tools. In the case of such thin coatings as these, c.a., 4-8 μm, the deposits are not required to have significant strength since their structural integrity is derived from the substrate upon which they are adherently deposited. Thin coatings of this kind which also were extremely fine-grained are described in U.S. Pat. No. 4,427,445 to Holzl.
The process described in the '345 patent and other related patents by Holzl is the most notable claim of using CVD as a means of producing metals, semi-metals, or refractory compounds having an unique combination of high-strength and excellent fracture toughness, especially in materials of high hardness.
Following the teachings of the aforementioned Hall-Petch relationship, there was reason to believe that the characteristic of the materials described in the '345 patent which caused them to have such unique properties was their extreme grain refinement, c.a., 50-100 nm. Their high hardness was attributed, at least in part, to their content of tungsten carbides. There is also ample reason to believe that the variability which was experienced in the products made by the method of the '345 patent was due to the presence of some irregular, larger grains in the structure.
In the specification of the '345 patent, Holzl postulated that the formation of the microcrystalline grain structure was a result of a reaction off of the surface of the substrate to form a liquid intermediate product which was subsequently reacted to form a second liquid intermediate product, which is deposited on the substrate, thence, rapidly, to be reacted to form the desired solid phase. In this respect, this process might be the equivalent of very rapid quenching of a metal from the melt which has been used to cast extremely fine-grain materials. Such a sequence of events probably occurred but, was not, in and of itself, sufficient to fully explain the results.
Holzl also postulated that the observed layered structure was caused by oscillating turbulence in flow of the fog or halo off of the substrate. This is now believed, based on the investigations of this invention, to have been an absolutely essential factor in the described deposition behavior.
The near impossibility of causing this oscillation to occur in a totally predictable way was most likely the fundamental cause for the process being non-reproducible and discarded as not commercially practical. Each time that the size or shape of the deposition reactor was changed and each time that the size or configuration of the work piece(s) was changed, an entirely new set of deposition conditions needed to be determined to establish this oscillating turbulence properly.
The process was so sensitive that even minor changes in the positioning of the work pieces in the reactor could cause failure of the processing runs. The layered structure was simply not acceptably uniform in its frequency and thickness of its layers.
The material made according to the method of the '345 patent was considered to be an alternate and improved method to the powder metallurgy of cemented carbides for the making of hard metal parts for tool and wear applications. It was considered to be superior to cemented carbides because it eliminated some of their deficiencies. In many cases, the wear resistance of cemented carbides is dictated more by the performance of the cement than by the hard particles and is thereby limited. In short, wear occurs frequently by the failure of the cement allowing the hard particles to be removed from the body without the particles, themselves, actually fracturing or wearing.
This behavior of cemented carbides can be compared with that of other wear materials like tool steels. Tool steels, although they contain two or more phases, wear like a homogenous material, not like a mixture. They also have greater toughness than any other materials of equivalent hardness.
If tool steels could be made as hard as the cemented carbides, they would be much preferred. The same statement could be made about cast, hard nickel or cobalt alloys versus the cemented carbides. The maximum hardness of tool steels or the cast hard alloys, however, is typically only about one half the maximum hardness of the cemented carbides; to wit, ˜7-9 GPa Vickers Hardness Number (HV) as compared to ˜11-22 GPa. For this reason they are disqualified from many applications for which cemented carbides can be used. An additional advantage of the cemented carbides over tool steels is, of course, their ability to maintain their strength and hardness at the high temperatures generated within the tool material in certain machining operations.
Note that as included in this disclosure, HV is used to denote Vickers Hardness Number as measured with a 500 g or 1000 g weight on a Shimadzu Microhardness Tester, unless otherwise cited.